The rate at which molecules from a gaseous or liquid phase can diffuse through a sheet of material, such as a foil, membrane, cloth, fabric etc., has a substantial importance in numerous practical applications, and consequently there exists a strong demand for instruments capable of providing quantitative information about this so-called permeation process. In the prior art there are a lot of techniques available that can measure the permeability of rather thick films, i.e. films with a thickness in the order of 1 μm and thicker, that can easily be isolated from a carrier or substrate on which they are deposited and that can be handled without the substrate or carrier.
Measurement of the permeability of membranes and foils by photoacoustic techniques is one of the possible techniques to measure permeability of rather thick films such as membranes, foils, etc. The central part of a photo acoustic membrane permeation measuring system is a measurement cell into which the membrane is inserted in such a way that it divides the cell into two separated volumes, i.e. the sample volume and the measurement volume. The component for which the permeation rate is to be measured is injected into the sample volume, either in a gaseous form having a well-controlled concentration in a buffer gas, or in a liquid form. The measurement volume is initially purged with a gas having a known (low or even zero) concentration of the component to be measured. The measurement volume is connected through gas tubing with a membrane pump and a photoacoustic cell in such a way that a completely closed volume is produced. The pump is controlled to be regularly switched on and the gas is mixed through the gas tubing allowing the measured component to enter the measuring photoacoustic cell. By plotting the photoacoustic signal as a function of time and with the help of a known receiving volume, the permeation rate can be calculated.
A lot of similar techniques are available in the state of the art to calculate permeability of membranes and/or films, but these alternatives are all limited to rather thick films with a thickness in the order of 1 μm and thicker, that can be handled easily. For thin films or (top) coatings having thicknesses in the order of several nm, which are deposited onto substrates and which cannot be separated or isolated from these substrates (meaning that they cannot be handled as a separate physical entity), these measurements are more complicated. Analytical tools that calculate the amount of solvent, e.g. water, absorbed by these films are described and are mostly based on mass change or solvent uptake, but there is a lack of techniques available to predict the permeability of these thin films or coatings towards certain solvents. Information on the diffusion rate of solvents through the thin film or coating can be very crucial e.g. for avoiding corrosion or leaching of compounds towards a substrate.
There are several areas wherein permeation of solvent molecules through a film plays an important role. One of the possible examples is immersion lithography applied in semiconductor industry and used as the key strategy to extend existing optical tools. A liquid, such as water, is placed between a lens and the photosensitive layers to enhance resolution. The role of liquids in contact with photoresist films is important; not only for component leaching and contamination, but also because of a possible influence of water on reaction and diffusion of photoacid generators. A thin top coating is usually deposited onto photosensitive layers to limit water migration to these photosensitive layers, also referred to as “resists”, underneath. These top coatings must meet multiple requirements, including efficiency as a barrier against leaching, a low amount of defects and transparency at 193 nm. In addition, they may not intermix with the photoresist, affect resist profiles or result in poorer resist performance than dry lithography.
Several attempts are made in literature to predict the water interaction of top coatings to be evaluated for immersion lithography. In “Metrology, Inspection, and Process Control for Microlithography XIX.”, Edited by Silver, Richard M. Proceedings of the SPIE, Volume 5753, pp. 508-518 (2005), the results of studies aimed at an improved understanding of how immersion in water during exposure influences the functional properties of films of lithographic materials are shown. Analytical techniques such as Quartz Crystal Microbalance (QCM), reflectance analysis of thin films and trace organic analysis are applied in this work. In QCM a mass/heat flow sensor is used.
The method described above and other current methods for measurement of solvent, e.g. water, interaction with top coatings are limited to information related to the amount of water uptake. They only reflect a mass change and therefore give no information on the real permeability kinetics. Furthermore, if more than one coating is present it will not be clear which layer is absorbing. Furthermore, the current methods available do not provide kinetics on the early (i.e. during the first seconds of contact) solvent uptake or, in other words, about the solvent uptake rate, which can be important information. Thus, current methods for determining solvent permeability in thin films or coatings that cannot be isolated from a carrier or substrate on which they are deposited have drawbacks and/or shortcomings. There is a need for improved methods determining solvent permeability through thin films and coatings.